The present invention relates generally to tensile testing of various material samples, to determine the suitability of modern pavement mixtures.
Road building and repair is an extremely expensive and disruptive task. In response to such concerns, Congress funded, in 1987, a research program meant to improve the durability and performance of United States roads. Longevity and safety are the primary directives of the research program. Obviously, roads which have a longer useful life reduce maintenance and reconstruction expenses. Roads which resist cracking, buckling, rutting and holing also have clear safety advantages.
Superpave(trademark) (a trademark of the Strategic Highway Research Program) is a product of research funded through this program. The pavement system optimizes asphaltic concrete pavement mixes to control undesired rutting, low temperature cracking and fatigue cracking. Development and installation of such pavements requires testing to obtain fundamental properties of the materials. Other construction techniques also require extensive material testing. Sampling of various concretes and other materials used in building construction ensures that engineering requirements and code requirements for the materials are maintained through building construction.
Tensile creep compliance, a temperature and loading time dependent stress-strain property, and tensile strength are important qualities of the asphaltic concrete mixtures developed under the highway initiative program. These properties are used with commercially available software to design and proportion asphalt concrete mixtures to be resistant to thermally-induced cracking. An Indirect Tensile Tester (IDT) has been approved by the Federal Highway Administration (FHWA), and has been the sole available test to provide required material property inputs for available thermal cracking performance prediction models contained in the FHWA""s Superpave(trademark) system and specification for designing high-quality asphalt paving mixtures.
The IDT is an expensive machine, costing approximately $140,000. This makes it one of the more expensive of the machines used to determine other characteristics associated with the design and installation of Superpave(trademark) mixtures. Related machinery such as a gyratory compactor, bending beam rheometer, dynamic shear rheometer and rotary viscometer, (all devices used to measure other various portions of mixture design specifications), have costs in the approximate $10,000 to $40,000 price range. The high price of the IDT is therefore a substantial barrier to its widespread use by contractors seeking to install Superpave(trademark) concrete mixtures.
Price concerns might eventually be addressed using the IDT, but a more significant difficulty associated with it is its large and cumbersome nature, and the relatively high level of expertise necessary to operate it properly. The IDT works under indirect tension test principles, where compressive loads are applied along thin loading strips on opposite sides of a cylindrical specimen being tested. This compressive loading creates tensile stress on opposite sides of the test specimen. As a result, up to 700 pounds per square inch of tensile stress is required to break asphalt concrete specimens at low temperatures. The need to generate that level of stress makes use of an approximately 22,000 pound capacity load frame and controller necessary. While these contribute significantly to the expense of the device, they also cause it to be large and cumbersome. The IDT frame weights 22,000 lbs. by itself, and is over seven feet tall. This effectively limits its use to a stationary laboratory, as opposed to transportation to a relevant field location. In the laboratory, it occupies a significant amount of space.
The IDT also has operational difficulties. Its associated test procedure requires the careful mounting of four high sensitivity transducers on every specimen. Every specimen must also be carefully aligned in the loading frame. A refrigeration based cooling system typically requires about fifteen minutes to restabilize before each test. Mounting the specimen, mounting and electrically balancing the transducers, and allowing appropriate time intervals between tests has shown that the testing is limited to about 1 sample per hour or less. In addition, a skilled technician is required for specimen mounting, transducer mounting and electrical balancing.
Subsequent to the development of the present invention, a new standard was introduced for determining the compliance of various asphaltic mixtures with static creep requirements. Namely, a 4xe2x80x3 cylinder is to be cored from a sample and appropriately tested. This is a radical departure from previous testing standards. Such tests can be performed on samples acquired in the field, however for purposes of testing new or raw materials such tests need to be performed on lab prepared specimens.
The need to obtain such specimens in the lab is not a new problem and various machines and methods presently exist that facilitate the creation of asphaltic samples. One such device is the Brovold Gyratory Compactor, manufactured and sold by Pine Instruments. Such a device produces a compacted cylindrical asphalt sample that may be tested for compliance with the various industry standards. The Gyratory Compactor will produce a cylindrical sample having an outer diameter greater than 4xe2x80x3. Thus to perform the above mentioned static creep test a 4xe2x80x3 cylinder is cored, thus leaving a cylindrical hoop. This is beneficial in that the compaction process will affect and modify the material at the outer circumference of the cylinder; thus the smaller cored sample has a more uniform density.
The difficulty is that asphalt mixtures are not homogeneous. Thus, the particular sample selected can vary the results. To perform all of the tests required, two samples (tensile creep compliance, static creep compliance) will need to be fabricated in the compactor. Thus, due simply to normal differences in the non-homogeneous asphalt mixture, the samples can be quite different. This makes use of the IDT (which now requires a second cylindrical sample) less desirable.
Furthermore, by its very nature the IDT can produce varying results on a given sample. As mentioned above, asphalt is not homogeneous. It is comprised of a plurality of particles ranging in size and configuration that are bonded together. The IDT effectively xe2x80x9csamplesxe2x80x9d selected particles and effectively ignores the remainder. Force is applied along two linear strips that are opposite one another on the cylinder, thus compressing the cylinder. This will cause the cylinder to deform into an oval or elliptical configuration. The mounted transducers monitor the change in diameter along the compressing direction and along the expanding direction. However, this is only done at selected points along the cross section (2 points for each direction). Thus, even though force is applied along the entire height of the cylindrical sample, its effects are only monitored at a few points. Thus, unless such a cylinder universally responds uniformly to an applied force, the IDT results can be affected. Furthermore, differences in the particulate nature of the asphalt can cause unique deformations to occur which will not be observed with the IDT due to its limited number of measurement points. Thus, the points selected for measurement will actually affect the results obtained. In other words, the IDT is incapable of averaging deformation across the whole sample.
The IDT thus suffers in that it cannot effectively measure average changes throughout a given sample and such tests cannot be performed on the same sample material used in the various other creep performance tests.
Accordingly, there is a need for an improved sample tester which addresses problems encountered in previous testers. It is an object of the present invention to provide such an improved sample tester capable of performing tensile creep compliance and tensile strength testing. The improved tester of the present invention has clear applicability to Superpave(trademark) testing procedures, as well as any construction or engineering analysis in which such material properties are relevant.
These and other needs and objects are met or exceeded by the present compact tensile tester. The compact device includes a frame that holds a hollow cylindrical sample around an inflatable membrane. The membrane is inflated via fluid pressure and the fluid pressure preferably is monitored to determine pressure at critical points in the testing procedures. Furthermore, characteristics of a pressure injector used to inflate the membrane are monitored. In a preferred structure, a piston and a cylinder pressure injector are operatively connected to the membrane and the injector is monitored for amount of piston travel. A pressure transducer monitoring fluid pressure, which correlates to the amount of force exerted on the sample is attached to the sample.
The structure includes shaped opposing platens to seal the membrane within a hollow cylinder sample being tested, a post failure restraint cylinder, and a cooling fluid bath. A computer can be connected to the pressure transducer to obtain values at specific critical testing points and calculate appropriate sample characteristics based upon the obtained values.
In operation, one of the platens is opened or removed to permit insertion of an appropriately dimensioned hollow cylindrical sample around the relaxed membrane. Due to the nature of the device, no careful alignment of the sample or sensors is required. Instead, the platen is closed, sealing the membrane in the inserted hollow cylindrical sample. While pressure in the membrane is monitored, it is inflated to place even hoop stress on the inner walls of the sample. In a sample failure test, the post failure restraint cylinder contains the sample. The device is quickly cycled for additional tests, in part due to its use of a relatively simple fluid cooling bath, but mainly due to the overall simplicity of the testing protocol and elimination of cumbersome positioning and sensor alignment procedures inherent in the prior art testing protocol.
In an alternative embodiment, various sensors are positioned on the hollow cylindrical sample to directly monitor the displacement. Thus, fluid pressure is used to exert a uniformly applied force across the whole of the specimen, but only point measurements are taken.
The present invention provides a method and apparatus for determining a sample""s average response to uniformly applied forces. As expected, this provides more accurate and complete data than is obtainable with the IDT. For this reason alone, the HCT becomes more advantageous to use than the IDT. Another advantage is the ability to perform the tensile tests with the HCT as well as the various static tests on a single asphalt sample made from one iteration in a Gyratory Compactor. As explained above, the compactor produces an asphalt cylinder. From this, a 4xe2x80x3 cylinder is cored and used for static testing. What remains is a hollow cylinder ideal for use in the HCT. Though the outer circumference of this hoop has been somewhat modified during the compaction process, this will have little or no effect on the HCT since force is being applied to the inner circumference and this is where a vast majority of the compression occurs. Thus, a single gyratory sample can be used for all of the various required tests.